1. Field of Invention
The present invention relates to ship buoyancy disinfection and biofouling treatment systems and techniques and, in particular, to utilizing oxidation reduction potential values to regulate and control electrocatalytic generation of chlorine-based oxidizing agents or biocides.
2. Discussion of Related Art
Chlorine based disinfection systems typically utilize any of dry chlorine gas, bulk sodium hypochlorite, and in-situ chlorine or sodium hypochlorite electrolytic generators. The electrolysis of seawater to produce chlorine has been used in land-based industrial and off-shore applications for biofouling control of cooling systems, such as systems that utilize seawater as a coolant. The development of self-cleaning tube-in-tube electrochemical cells has resulted in use of electrochlorination in shipboard applications, such as for biofouling control of engine cooling system, and air conditioning and other auxiliary systems.
A typical system layout for a land based chlorination system is schematically presented in FIG. 1A. Seawater is taken from a water intake or source 1 and pumped through an electrolytic generator 3 by a pump 2. The outlet of generator 3 containing a biocidal agent is delivered into a storage tank 5. A power supply 4 provides electrical current to electrolytic chlorine generator 3.
Storage tank 5 is typically equipped with one or more air blowers 6 that provide dilution or dispersion of a hydrogen gas by-product to a safe concentration. Direct hydrogen removal can be effected with hydrocyclones instead of the air blowers and tanks. Land based systems can produce hypochlorite solutions at relatively high concentrations, in a range of about 500 ppm to 2,000 ppm chlorine. One or more dosing pumps 7 can be utilized to dose chlorine to a point of use typically by way of a distribution device 8. The point of use is typically an intake basin which provides water to another process such as, but not limited to, a cooling loop 9.
In some applications, dechlorination systems and techniques may utilize an oxidizer-neutralizing agent, such as sodium bisulfite, for downstream treatment of the potable water or cooling water, prior to discharge or use thereof.
Ships use ballast water tanks to provide stability and maneuverability. Typically, ballast tanks are filled with water at one port after or during cargo unloading operations. The ballast water may be discharged at another port if cargo is loaded. Effectively, the ballast water would be transferred from the first port to the second port, with a potential for the introduction of aquatic nuisance species (ANS) at the second port. ANS transfer can be a detrimental ecological issue. Shipboard electrochlorination systems, as schematically illustrated in FIG. 1B, are typically configured for low chlorine output with direct injection of chlorinated water. In shipboard electrochlorination systems seawater is typically delivered from a sea chest 1 or a main using a booster pump 2 to an electrolytic generator 3. Generator 3 is typically powered by a power supply 4. A product stream from generator 3 is typically injected into sea chest 1 through a distribution device 8. In shipboard systems, cooling water is typically discharged outboard D and can be de-chlorinated by introducing an neutralizing agent, such as sodium bisulfite, from a source 11 to reduce the chlorine concentration therein to an allowable discharge level, typically less than 0.1 ppm.
Typically, a chlorine analyzer is utilized to monitor and maintain a concentration of residual chlorine in treated water. Such systems, however, do not consider variabilities in chlorine demand in different ports where ballasting operations may occur. For example, chlorine demand may be affected by the concentration of nitrogen compounds in seawater, which may vary significantly from port to port and from season to season due to, for example, an algae bloom. FIG. 2 shows seasonal variations in ammonium and nitrate levels in the North Sea off the Texel Island, the Netherlands (provided by the Royal Netherlands Institute for Oceanographic Research (NIOZ)). The fluctuations in chlorine demand can create a higher than desirable or acceptable oxidizer concentration, such as a high free chlorine concentration, in the various shipboard systems which, in turn can accelerate or even promote corrosion of the ship systems and ancillary unit operations such as but not limited to ballast water pumps, piping, and tanks. The variabilities can also promote formation of disinfection byproducts (DBP).